The present invention is directed to an integrated system, process and apparatus for the recovery of chemical values from spent pulping liquors emanating from the paper industry.
In processes involving chemical pulping of lignocellulosic materials, such as wood chips, the common processes are usually referred to as the kraft process and the sulfite process. There are some processes which employ chemicals along with mechanical steps, and these processes are sometimes referred to as semi-chemical or chemi-mechanical pulping processes. These processes use some o the same chemicals as the kraft and sulfite processes. Another process is the so-called soda process.
The preferred chemical pulping process is the so-called kraft process. The kraft process involves cooking or pulping the wood chips in an alkaline aqueous solution of sodium hydroxide, sodium carbonate, and sodium sulfide. This is usually carried out in a pressure vessel called a digester in which the contents are heated to temperatures of about 160.degree. to 180.degree. C., for about 1 to 3 hours. At the end of the cooking or pulping stage, the kraft cooking liquor is washed from the pulp and is subjected to a recovery treatment to recover the chemical and energy values. This material is known as kraft black liquor because of its dark color.
In the so-called sulfite process, the wood chips are pulped or cooked in an acidic aqueous solution of sulfur dioxide, along with chemicals providing calcium, magnesium, sodium, or ammonium ions. Essentially, the aqueous solution is made up of sulfurous acid and sulfite and bisulfite ions. In this process, the cooking period lasts from about 6 to 8 hours during which the temperature rises to about 140.degree. C. In a variation of the sulfite pulping process, the cooking liquor, instead of being an acidic solution, is neutralized or made mildly alkaline. The spent liquor from the conventional sulfite pulping processes have been recovered in several ways. In general, it has been more difficult to recover the chemical values from the spent liquors of the sulfite process than from the black liquors of the kraft process. This is partially responsible for the predominant use of the kraft process over the sulfite process in the pulping of lignocellulosic material.
Heretofore, the traditional process for the recovery of kraft black liquor has been the use of the so-called Tomlinson kraft recovery boiler. In the Tomlinson recovery boiler, concentrated black liquor serves as fuel to provide heat for general process use. The combusted liquor produces, in addition to the heat values from combustion of the organic matter present, a smelt, or molten body of inorganic chemicals. The smelt is dissolved in water to produce so-called green liquor. This is an aqueous solution of sodium carbonate and sodium sulfide. This green liquor, after treatment with lime (calcium hydroxide) to convert some of the sodium carbonate to sodium hydroxide, becomes white liquor (after removal of by-product calcium carbonate by filtration). The white liquor is a cooking or pulping liquor which can be used in future kraft pulping operations after replacement of any depleted components.
The use of the Tomlinson recovery furnace presents a number of problems. At the outset, its use presents an opportunity for serious explosions if water inadvertantly contacts molten inorganic salts. Another disadvantage is that the recovery of energy values from the spent kraft black liquor is substantially less than desirable. A further disadvantage is that the Tomlinson process does not permit independent control of the physical and chemical actions which are present within the process. Also, the process tends to cause pollution of the atmosphere by reduced sulfur compounds.
It is, therefore, not surprising that the paper industry has sought other more satisfactory means of waste pulping liquor recovery. One such effort has been that described in Flood U.S. Pat. No. 3,322,492. In that process a sequence of at least two fluidized bed operations was employed. In the first fluidized bed chamber, black liquor is dried to solid granules by heated air. In the second fluidized bed chamber, the dried solid granules produced in the first chamber are subjected to further heating under conditions which cause decomposition of the organic constituents and some reduction of the sodium sulfate content by virtue of the carbon monoxide derived from the decomposition of the organic matter. This forms sodium sulfide and carbon dioxide. Off gases from this process include hydrogen sulfide, along with carbon dioxide, carbon monoxide, hydrogen, and nitrogen.
In the process of Shal U.S. Pat. No. 3,574,051, kraft black liquor is concentrated by contact with a stream of heated air under conditions which do not cause decomposition of the black liquor constituents. The resulting concentrated black liquor is then burned with air in a fluidized bed reactor, in an excess of air, so as to produce total oxidation of the black liquor components, both organic and inorganic. Under these conditions, all sulfur compounds are converted primarily to sodium sulfate. The resulting inorganic solid salts are discharged from the fluidized bed reactor in which the main constituents are sodium sulfate and sodium carbonate. These solid salts are then passed through to a chemical reactor and subjected to a reducing gas stream which is made up of carbon monoxide as the reducing component. This causes the sodium sulfate in the solid salts to be reduced to sodium sulfide, and sodium sulfide along with sodium carbonate are dissolved in water to form green liquor which can then be processed in the usual manner to provide white liquor.
Osterman et al. U.S. Pat. No. 3,523,864 describes a process for the recovery of kraft black liquor involving the treatment of dried black liquor in a three-zone fluidized bed reactor in which black liquor is dried and oxidized in the intermediate zone, the inorganic sodium and sulfur compounds are reduced in the bottom zone, and the calcium carbonate is calcined to form calcium oxide in the top of the three zones.
Priestley U.S. Pat. No. 3,578,396 describes a process for reclaiming chemicals from spent sulfite pulping liquors by reacting the spent liquor in a thermal oxidizing fluidized bed reactor to produce, at a temperature of 1200.degree. to 1400.degree. F., a granular material, while still in a fluidized bed reactor, which is cooled somewhat and subjected to treatment with sulfur dioxide at a temperature of approximately 1000.degree. to 1200.degree. F. This is said to convert some of the carbonate to sodium acid sulfate.
Shick U.S. Pat. No. 3,676,064 is another patent describing a process for the recovery of chemical values from spent sulfite pulping liquors. In that process, the spent sulfite liquor is treated in a fluidized bed reactor to convert the inorganic solids to essentially sodium sulfate and sodium carbonate, and the inorganic material is pyrolysed. Sulfur dioxide gas is absorbed into a soluton of sodium carbonate produced in the fluidized bed reactor.
Copeland U.S. Pat. No. 3,864,192 describes a process for the recovery of sulfite spent pulping liquors whereby the spent liquor is combined with magnesia, the mixture concentrated by evaporation at an elevated temperature, and the concentrated spent liquor is fed to a fluidized bed reactor and combusted therein.
Copeland U.S. Pat. No. 3,862,909 describes a process for recovering chemical values from kraft or sulfite spent pulping liquors by controlled autogeneous combustion or pyrolysis of the concentrated liquors in a fluidized bed reactor in which the carbonaceous material is converted to activated carbon. The inorganic salts are leached with water and thereafter recovered.
Copeland et al. U.S. Pat. No. 3,309,262 describes a process for the recovery of spent pulping liquors, including kraft black liquor. The spent liquor is concentrated and introduced by atomization into a fluidized bed reactor. The resulting waste liquor spray encounters residual inorganic chemicals derived from the combustion of previous spent liquors. Such chemicals are, for example, sodium sulfate, sodium carbonate, sodium hydroxide, and mixtures thereof. Additionally, the fluidized bed reactor may contain other and different, inert materials, such as silica grains in admixture with the inorganic chemicals. In the fluidized bed reactor, the organic material is combusted. The resulting dry granular inorganic material produced is to some extent recycled or retained in the fluidized bed reactor for further operations and some of the inorganic material is then subjected to recovery.
The use of conventional fluidized bed combustors, as in the case of the foregoing patent, has been plagued by the inability to remove heat of combustion at temperatures in the order of 1300.degree. F., which temperatures occur in a normal combustion operation. This is due to poor mixing, corrosion of the equipment, and fouling of the cooling tubes.
As far as is known, none of these foregoing processes has ever achieved utilization on a commercial scale.
It is, therefor, an object of the present invention to provide a system, process and apparatus for the efficient recovery of energy and chemical values in spent pulping liquors.
It is a further object of the present invention to provide a system, process and apparatus for the recovery of chemical values from spent pulping liquors from various pulping processes which are free from the disadvantages of the prior art recovery means.
It is a further object of the present invention to provide a system, process and apparatus for separation and individual control over each of the three prime functions of a kraft recovery system, viz., combustion, steam generation and sulfate reduction.
Other objects of the present invention will be apparent to those skilled in the art from the present description, taken in conjunction with the appended drawing which is a flow sheet or flow diagram describing the system, process and apparatus of the present invention.